Optical imaging systems and methods which provide real-time detection, diagnosis and imaging of diseases such as cancer are known in the art. The application of such systems in vivo during real time medical or surgical procedures has been limited by a poor signal to noise ratio. This low signal to noise ratio is a consequence of the low strength or absence of an optical signal arising from the target tissue, the high level of background noise from any ambient light, and the poor sensitivity and specificity of the detected optical signal.
The magnitude of the signal to noise problem is best appreciated by considering that the reflected irradiance from tissue surfaces in an operating room can be as high as 10 mW/cm2, or more, under the brilliant surgical headlamps and overhead spotlights. Under normal operating room illumination, this translates into 1018 (1 billion billion) photons reflected from each square centimeter of tissue per second. In contrast, the native fluorescence may be on the order of 107 below the incident light, about 109 photons/cm2/sec, or less. This makes the detection of trace amounts of disease difficult in ambient light. Using the photon counts in the example just described, the ambient light is one billion fold greater per square centimeter than the detectable native fluorescent signal from a 1 mm tumor, or one trillion fold greater per square centimeter than the signal from a 100 micron tumor. Complicating matters further, some target tissues or disease conditions have a signal that is non-specific, or may have no detectable optical signal at all.
Detection of trace amounts of target tissue in room light therefore requires that the signal to noise ratio be improved beyond what is currently taught in the art, in the range of 104 to 109 fold improvement, either by reducing the noise introduced by background ambient light, or by increasing the strength and specificity of the target signal, or both.
With regard to improving the signal to noise through the rejection of ambient light, the majority of known optical diagnostic systems and methods simply ignore, background subtract, or turn off room light. Examples include most invasive devices equipped with optics, such as catheters, needles, and trocars (e.g., U.S. Pat. No. 5,601,087), as well as noninvasive devices for imaging or measuring the optical features of living tissue externally (e.g., U.S. Pat. No. 5,936,731, WO 98/10698, Sweeny et. al. in Proc. Natl. Acad. Sci. 1999;96(21):12044-12049, Weissleder et. al. in Nature Biotech 1999;17:375-378). However, turning off the lights or obscuring a view of the patient using dark drapes during a medical procedure is disadvantageous and possibly dangerous, especially during critical surgical procedures. On the other hand, simply ignoring or background subtracting ambient light risks overflowing the detector and/or burying weak signals from trace amounts of tissue in the background noise. Thus, such simplistic approaches diminish the possibility of real-time feedback during medical procedures, unless the ambient light is specifically rejected.
Schemes for the true rejection of ambient light are known, and include the use of band pass filters, temporal signal modulation, and lifetime analysis. However, such methods often fail in ambient light. For example, a bandpass filter wide enough to pass the bulk of a biological fluorescent signal in vivo will still pass at least 5% of the ambient light as well, far short of the needed 104 to 109 enrichment in signal to noise required to unmask a weak native fluorescence. Temporal methods can be arranged to provide background rejection in tissue imaging, however approaches such as amplitude modulation (e.g., U.S. Pat. Nos. 5,213,105, 5,648,269, and 5,865,754) and time-of-flight measurement (e.g., U.S. Pat. No. 5,919,140, WO 98/10698) have historically been configured instead to measure temporal information about the time required for photons to traverse a tissue or leave a fluorescent molecule, rather than to reject ambient signal. In addition, such temporal schemes work well only when the target signal is sufficiently strong that a temporal response (time-of-flight rise or fall time, phase shift, fluorescence lifetime) can be accurately estimated, and thus fail to operate in real time when only trace amounts of the target tissue are present, or when the ambient light is strong.
All of the above systems and methods fail to reject significant amounts of background radiation, operate only under darkened conditions, require strong target signals, or require long integration times that preclude real-time use, and thus fail to reliably detect trace amounts of a target tissue in the presence of common levels of ambient light.
The above drawbacks notwithstanding, improved ambient light rejection alone may not be sufficient to achieve a real-time system for use in vivo under operating room conditions. Although some large cancers may be detectable using native signals (e.g., U.S. Pat. No. 5,647,368), many physiological and pathological conditions possess no detectable native optical signal, or have only a weak or non-specific optical signature, particularly when trace amounts of disease are to be detected.
Contrast agents have been used in the past for medical monitoring and imaging when the inherent or native signal in vivo is absent or poor. A contrast agent serves to lend a strong, identifiable signal to an otherwise poorly detectable tissue. In this regard, optical contrast agents are known in the art. The majority of known optical contrast agents are untargeted (e.g., U.S. Pat. No. 5,672,333, U.S. Pat. No. 5,928,627, WO 97/36619, Lam et al. in Chest 1990:97(2):333-337, and Hüber et. al. in Bioconjugate Chem 1998;9:242-249). Such untargeted agents rely largely upon the physical characteristics of the agent, such as solubility in fat versus water, or upon cellular metabolism, to non-specifically partition them into a particular tissue. As untargeted dyes produce widely distributed and nonspecific signals, they require a large stained tissue volume in order to generate a statistically clear signal in vivo, they tend to produce weak signals, insufficiently above background noise, from smaller or trace target tissue volumes. For example, photodynamic therapy agents accumulate in certain cancerous cells (Lam et. al., Huber et. al.), but they also accumulate in normal tissues. None of the preceding optical contrast methods or devices teach in vivo targeting of rare cells through the use of highly specific targeting agents, and thus will fail for use in the detection of trace amounts of a target tissue in vivo.
A few agents allowing for more localization targeting of dye are known. For instance, Sweeny et. al. teach a process of genetically altering cells cultured or grown outside of the body, followed by insertion of these cells into the body for imaging. Such removal and reinsertion of cells may not make sense for use in patients with spontaneous diseases such as cancer, which should not be altered and reintroduced into the patient's body. Further, as the transformed cells glow continually, there is no inducible component to the light (such as fluorescent light induced using a flash lamp), and therefore little if any enhancement is possible through use of gated rejection of ambient light. Weissleder et. al. (Nature Biotech 1999;17:375-378) teach another dye that is targeted in the sense that it is locally activated, while WO 98/48846 suggests multiple methods of creating optical dyes for use in vivo. Each of these approaches is realized without provision for the specific rejection of ambient light though use of a gated detector, and thus will fail for use in the detection of trace amounts of a target tissue in vivo and in the presence of ambient light.
In summary, none of the preceding approaches suggest gating the detector to substantially reject ambient light, nor do they suggest combining a gated ambient-light-insensitive imager, a high-power synchronized light source, and highly-specific tissue-targeted contrast agent into a cohesive system for detecting and imaging trace amounts of target tissue in room light. Therefore, device and methods taught in the present art will fail in many cases to detect and localize trace disease such as small primary cancers, early metastatic disease, local inflammatory conditions, or unstable coronary wall plaque, in vivo and in real time under ambient light conditions. A real-time optical system and method to detect or image an induced or contrast-influenced target tissue signal in vivo and in ambient light has not been taught, nor has such a tool been successfully commercialized.
What is needed, and not yet suggested or taught, is a method and optical system to detect a signal from trace tissue in vivo and in real time despite the presence of a high level of ambient light, possibly including the step of enhancing a weak native signal (or creating one where one did not previously exist), in order to produce an optical system for detecting, imaging, targeting, and treating of tissue, such as trace amounts of cancer, in vivo and in real time.